This invention relates generally to systems for reading characters and more particularly to an ink formulation of the sympathetic type which are used to mark characters to identify machine readable characters having a need of a correction.
In particular the ink is utilized in order to print the marking dot which is described, for example, as a disappearing ink spot, as in U.S. Pat. application Ser. No. 573,787, filed May 1, 1975 now U.S. Pat. No. 4,068,212 granted Jan. 10, 1978 entitled, "Method and Apparatus for Identifying Characters Printed on a Document which can not be Machine Read", said application being incorporated herein by reference in its entirety.
As mentioned in the application referred to above, ink from a reservoir is applied to a document to cause a marking dot to appear adjacent an unsatisfactory character. If the document may be handled by humans at a later time after correction of the character, the presence of the marking dot may cause confusion since the viewer will be unable to determine whether or not the character which appears to be machine readable has been corrected. It was suggested in the aforesaid application that it would be desirable to use a "disappearing" ink. Such a sympathetic ink will have a color change to an invisible shade or a slightly visible or invisible shade shortly after the document leaves an inspection-correction station. In order to aid the operator in identifying a character to be corrected as each document which passes a read station is inspected electrically and after determination is made that the document cannot be read the "can't read" mark is printed with the sympathetic ink adjacent the unreadable character. This identifies the character which must be corrected by an operator.
The requirements of this sympathetic ink are that it be of high saturation so as to clearly identify the unsatisfactory character yet it cannot mar the document nor can the document be discolored by subsequent handling. The ink should disappear shortly after the time required to correct the document.